Strength and toughness can be provided to composites through use of embedded reinforcing fibers. Such structures have been reported extensively in the patent and other literature. However, for composites planned for use in an oxidizing environment at temperatures greater than 1000.degree. C., there are limited numbers of materials for the fiber and for the matrix in which the fiber is embedded. With various combinations of such materials, there are adverse events which can occur. For example, detrimental chemical interaction, oxidation or undesirable (i.e., too strong or too weak) bonding between the fiber and the matrix can reduce composite toughness and the effective and practical use of the composite.
Because the interface between a fiber and its surrounding matrix is important in reinforced composites, coatings have been proposed as a transition medium. A number of studies reported on this subject include "Fiber Coating and Characterization" by David C. Cranmer, CERAMIC BULLETIN, Vol 68, No 2, 1989, pages 415-419; and "The Role of the Fiber-Matrix Interface in Ceramic Composites" by Ronald J. Kerans et al, CERAMIC BULLETIN. Vol 68, No 2, 1989 pages 429-442. For the purpose of background and including herein an explanation of the mechanisms involved, the text of those articles are hereby incorporated herein by reference.
As discussed in those articles and elsewhere in the literature, attempts to control interface properties are now in their infancy. However, it has been observed that relatively weak interface layers can be beneficial, for example, in respect to the mechanism of fiber and/or matrix fracture. Having fiber reinforcements of higher strength and modulus than the matrix can increase properties of the composite if a load can be transferred properly from the matrix to the fiber. It is apparent that an interface coating, having a composition and microstructure which is preselected to be beneficial to the matrix-fiber combination, can provide improved strength and toughness to composites. This is especially important for use in an oxidizing environment at temperatures greater than 1000.degree. C.: exposure to such temperatures increase the potential for fiber degradation as well as reaction between the materials in contact at the interface, for example fiber with coating and coating with matrix.
It has been observed that, in some cases, a relatively fine microstructure, referred to in such terms as crystalline, microcrystalline or crystalites, can provide such a beneficial coating structure. In other cases it is believed that a microstructure of relatively coarse interlocking crystals can be beneficial.